1. Technical Field
This invention generally relates to improving sound system performance in a given space. More particularly, the invention relates to improving the frequency response performance for one or more listening positions in a given area thus providing a more enjoyable listening experience.
2. Related Art
Sound systems typically include loudspeakers that transform electrical signals into acoustic signals. The loudspeakers may include one or more transducers that produce a range of acoustic signals, such as high, medium and low-frequency signals. One type of loudspeaker is a subwoofer that may include a low frequency transducer to produce low-frequency signals.
The sound systems may generate the acoustic signals in a variety of listening environments. Examples of listening environments include, but are not limited to, home listening rooms, home theaters, movie theaters, concert halls, vehicle interiors, recording studios, and the like. Typically, a listening environment includes single or multiple listening positions for a person or persons to hear the acoustic signals generated by the loudspeakers. The listening position may be a seated position, such as a section of a couch in a home theater environment, or a standing position, such as a spot where a conductor may stand in a concert hall.
The listening environment may affect the acoustic signals, including the low, medium and/or high frequency signals at the listening positions. Depending on where a listener is positioned in a room, the loudness of the sound can vary for different tones. This may especially be true for low-frequencies in smaller domestic-sized rooms because the loudness (measured by amplitude) of a particular tone or frequency may be artificially increased or decreased. Low frequencies may be important to the enjoyment of music, movies, and most other forms of audio entertainment. In the home theater example, the room boundaries, including the walls, draperies, furniture, furnishings, and the like may affect the acoustic signals as they travel from the loudspeakers to the listening positions.
The acoustic signals received at the listening positions may be measured. One measure of the acoustical signals is a transfer function that may measure aspects of the acoustical signals including the amplitude and/or phase at a single frequency, a discrete number of frequencies, or a range of frequencies. The transfer function may measure frequencies in various ranges.
The amplitude of the transfer function indicates the loudness of a sound. Generally, the amplitude of a single frequency or a range of frequencies is measured in decibels (dB). Amplitude deviations may be expressed as positive or negative decibel values in relation to a designated target value. When amplitude deviations are considered at more than one frequency, the target curve may be flat or of any shape. An amplitude response is a measurement of the amplitude deviation at one or more frequencies from the target value at those frequencies. The closer the amplitude values measured at a listening position correspond to the target values, the better the amplitude response. Deviations from the target reflect changes that occur in the acoustic signal as it interacts with room boundaries. Peaks represent an increased amplitude deviation from the target, while dips represent a decreased amplitude deviation from the target.
These deviations in the amplitude response may depend on the frequency of the acoustic signal reproduced by the subwoofer, the subwoofer location, and the listener position. A listener may not hear low-frequencies as they were recorded on the recording medium, such as a soundtrack or movie, but instead as they were distorted by the room boundaries. Thus, the room can change the acoustic signal that was reproduced by the subwoofer and adversely affect the frequency response performance, including the low-frequency performance, of the sound system.
Many techniques attempt to reduce or remove amplitude deviations at a single listening position. One such technique comprises global equalization, which applies filters equally to all subwoofers in the system. Generally, the amplitude is measured at multiple frequencies at a single position in the room. For example, an amplitude measurement may be taken at 25, 45, 65, and 80 Hz to give an amplitude deviation for each measured frequency. Global equalization may comprise applying filters at each of the subwoofers to reduce a +10 dB deviation at 65 Hz. Global equalization may thus reduce amplitude deviations by either reducing the amplitude of the frequency range having positive deviations from the target or boosting the output of the subwoofers at the frequency range having the greatest negative deviation from the target. Global equalization, however, may only correct amplitude deviations at a single listening position.
Another technique which attempts to reduce or remove amplitude deviations is spatial averaging. Spatial averaging, which is a more advanced equalization method, calculates an average amplitude response for multiple listening positions, and then equally implements the equalization for all subwoofers in the system. Spatial averaging, however, only corrects for a single “average listening position” that does not exist in reality. Thus, even when using spatial averaging techniques, some listening positions still have a significantly better low-frequency performance than other positions. Moreover, attempting to equalize for a single location potentially creates problems. While peaks may be reduced at the average listening position, attempting to reduce the dips requires significant additional acoustic output from the subwoofer, thus reducing the maximum acoustic output of the system and potentially creating large peaks in other areas of the room.
Apart from equalization and spatial averaging, prior techniques have attempted to improve the sound quality at a specific listening position using loudspeaker positioning. One technique analyzes standing waves in order to optimize the placement of the loudspeakers in a room. Standing waves may result from the interaction of acoustic signals with the room boundaries, creating modes that have large amplitude deviations in the low-frequency response. Modes that depend only on a single room dimension are called axial modes. Modes that are determined by two room dimensions are called tangential modes and, modes that are the result of all three room dimensions are called oblique modes.
FIG. 1 is a pictorial representation of the first four axial modes for a single room dimension for an instant in time. Sound pressure maxima exist at the room boundaries (i.e., the two ends in FIG. 1). The point where the sound pressure drops to its minimum value is commonly referred to as a “null.” If there is no mode damping at all the sound pressure at the nulls drops to zero. However, in most real rooms the response dip at the nulls are in the −20 dB range. As shown in FIG. 1, standing waves may have peaks and dips at different positions throughout the room so that large amplitude deviations may occur depending on where a listener is positioned. Thus, if listener C is positioned in a 30 Hz peak, any 30 Hz frequency produced by the subwoofer will sound much louder than it should. Conversely, if listener D is positioned in a 30 Hz dip, any 30 Hz frequency produced by the subwoofer will sound much softer than it should. Neither corresponds to the acoustic signal reproduced by the subwoofer or previously recorded on the recording medium.
There are several methods to reduce standing waves in a given listening room through positioning of loudspeakers. One method is to locate the subwoofer at the nulls of the standing waves. Specifically, the loudspeaker and a specific listening position may be carefully selected within the room so that the transfer function may be made relatively smooth at the specific listening position. A potential loudspeaker-listener location combination is shown in FIG. 2 with the first four axial modes along the length of the room. The specific listening position may be located away from the maxima and nulls for the first, second and fourth order modes, while the loudspeaker may be located on the null of the third order mode. As a result, if these are the only resonant modes in the room, this specific listening position should have a relatively smooth transfer function. However, this method merely focuses on a single, specific listening position in order to reduce the effects of standing waves in the listening environment; it does not consider multiple listening positions or a listening area. In practice, the presence of other axial, tangential, and oblique room modes make prediction using this method unreliable.
Another method is to position multiple subwoofers in a “mode canceling” arrangement. By locating multiple loudspeakers symmetrically within the listening room, standing waves may be reduced by exploiting destructive and constructive interference. However, the symmetric “mode canceling” configuration assumes an idealized room (i.e., dimensionally and acoustically symmetric) and does not account for actual room characteristics including variations in shape or furnishings. Moreover, the symmetric positioning of the loudspeakers may not be a realistic or desirable configuration for the particular room setting.
Still another technique to configure the audio system in order to reduce amplitude deviations is using mathematical analysis. One such mathematical analysis simulates standing waves in a room based on room data. For example, room dimensions, such as length, width, and height of a room, are input and the various algorithms predict where to locate a subwoofer based on data input. However, this mathematical method does not account for the acoustical properties of a room's furniture, furnishings, composition, etc. For example, an interior wall having a masonry exterior may behave very differently in an acoustic sense than its wood framed counterpart. Further, this mathematical method cannot effectively compensate for partially enclosed rooms and may become computationally onerous if the room is not rectangular.
Another mathematical method analyzes the transfer functions received at the listening positions and solves for equal transfer functions received at the listening positions. FIG. 3 illustrates an example of a multi-subwoofer multi-receiver scenario in a room. Reference 1 is the signal input to the system. The loudspeaker/room transfer functions from loudspeaker 1 and loudspeaker 2 to two receiver locations in the room are shown as H11 through H22, while R1 and R2 represent the resulting transfer functions at two receiver locations. Each source has a transmission path to each receiver, resulting in four transfer functions in this example. Assuming the signal sent to each loudspeaker can be electrically modified, represented by M1 and M2, the modified signals may be added. Here, M is a complex modifier that may or may not be frequency dependent. To illustrate the complexity of the mathematical solution, the following equations solve a linear time invariant system in the frequency domain:R1(f)=IH11(f)M1(f)+IH21(f)M2(f)R2(f)=IH12(f)M1(f)+IH22(f)M2(f)  (1)where all transfer functions and modifiers are understood to be complex. This is recognized as a set of simultaneous linear equations, and can be more compactly represented in matrix form as:
                                                        [                                                                                          H                      11                                                                                                  H                      21                                                                                                                                  H                      12                                                                                                  H                      22                                                                                  ]                        ⁡                          [                                                                                          M                      1                                                                                                                                  M                      2                                                                                  ]                                =                      [                                                                                R                    1                                                                                                                    R                    2                                                                        ]                          ,                            (        2        )            or simply,HM=R,  (3)where the input 1 has been assumed to be unity.
A typical goal for optimization is to have R equal unity, i.e. the signal at all receivers is identical to each other. R may be viewed as a target function, where R1 and R2 are both equal to 1. Solving equation (3) for M (the modifiers for the audio system), M=H−1, the inverse of H. Since H is frequency dependent, the solution for M must be calculated at each frequency. The values in H, however, may be such that an inverse may be impossible to calculate or unrealistic to implement (such as unrealistically high gains for some loudspeakers at some frequencies).
As an exact mathematical solution is not always feasible to determine, prior approaches have attempted to determine the best solution calculable, such as the solution with the smallest error. The error function defines how close is any particular configuration to the desired solution, with the lowest error representing the best solution. However, this mathematical methodology requires a tremendous amount of computational energy, yet only solves for a two-parameter solution. Acoustical problems that examine a greater number of parameters are increasingly difficult to solve.
Therefore, a need exists for a system to accurately determine a configuration for an audio system such that the audio performance for one or more listening positions in a given space is improved.